Several processes have been reported heretofore for producing acrylamide by means of catalytic hydration. Typical of such processes are those disclosed in U.S. Pat. No. 3,381,034, in which a cuprous ion is employed; in U.S. Pat. No. 3,597,481, in which an oxide of silver, zinc or cadmium and an oxide of chromium are employed; in U.S. Pat. No. 3,631,104, in which copper oxide, copper-chromium oxide, copper-molybdenum oxide or a copper catalyst prepared by reducing any one of such oxides are employed; in U.S. Pat. No. 3,674,848, in which a I-B or II-B group metallic salt of acid cation exchange resin is employed; and in U.S. Pat. No. 3,673,250, in which a homogeneous catalyst consisting of organic phosphines or the like and transition metallic compounds are employed. Another example involves a process which is carried out by use of Raney copper, Ullmann copper, reduced copper or a catalyst made of substantially any one metal selected from a class including silver, gold and copper with a carrier (U.S. Pat. Application, Ser. No. 56,967, filed on July 21, 1970 and owned by the assignee of the present application). To date, however, no process suitable for industrial use is known for effectively concentrating an acrylamide aqueous solution by evaporating the water content while controlling the acrylamide polymer content in the acrylamide crystal or in the concentrated acrylamide aqueous solution product to less than 0.2 weight % in terms of butanol-insoluble residue.
In connection with the storage of such a vinyl monomer as liquid acrylic alkyl ester or the like, it is known that the vinyl monomer can be prevented from polymerizing by saturating it with air and/or oxygen. However, in a process wherein an acrylamide aqueous solution is concentrated by distillation at a high temperature, polymerization could occur more readily. Thus, it may be impossible to maintain the acrylamide polymer content in the resulting acrylamide crystals or concentrated acrylamide aqueous solution at a level less than 0.2 weight % merely by saturating the solution to be distilled with air and/or oxygen, or by applying the method of the conventional sulfuric acid process for producing acrylamide.
It may be clear to those skilled in the art that the technique for limiting the acrylamide polymer content to a level less than 2.0 weight % is extremely important when acrylamide crystals or an acrylamide-rich aqueous solution are used in the form of acrylamide monomer, for example, as a paper reinforcing agent.